The present invention relates to oceanographic solid-state sensors and detectors, and more particularly to a sensor system having means for self-cleaning the surface of foreign matter and also for bio-fouling prevention of the surface by use of acoustic energy.
Until now, certain oceanographic measurements have been hindered due to the problem of marine organisms in ocean water attaching to the surface of submerged solid-state measuring devices. Frequently, it is desired to have oceanographic sensors remain submerged in the ocean for 6 months or longer. It is not practical to frequently remove a sensor or detection device from the water for cleaning purposes. The only methods available in the past to prevent fouling of the sensor devices employed the use of toxic materials which were adhered as a coating on the surface of the sensor. The anti-fouling coating afforded some protection for a limited period of time, but also reduced the sensitivity of the sensor and its general utility.
Oceanographic sensors in general have a limited in-situ life due to the major problems of bio-fouling and contamination (bio-fouling being defined as organic growth on the sensor's surface while submerged in the ocean). To alleviate this problem, the present invention provides a system for in-situ cleaning of the bio-fouling/contamination and also for bio-fouling prevention. It entails the use of sonic energy which varies in frequency from approximately 10 KhZ to frequencies which approach the resonant frequency of the organic matter. It also varies in energy intensity levels, from levels which create acoustic streaming forces to intensities which cause cavitation. Acoustic streaming forces are those forces, caused by movement of water adjacent to the transducer or surface to which the transducer is attached, which are generated by the motion of the sensor transducer prior to cavitation. Cavitation forces are those forces which are created as the pressure at the surface of the sensor is reduced to a point where the water is boiled and creates cavitation bubbles.
The present invention is an oceanographic sensor, cleaning transducer and bio-fouling prevention system. The sensor elements are fabricated on a transducer (e.g., piezoelectric or magnetostrictive) substrate which allows in-situ cleaning and fouling prevention of marine organisms and other foreign matter from its surface and thereby prevents reduction of sensitivity of the sensor. The sensor core is fabricated from a crystal substance and the sensing element is usually fabricated on a surface of the core. When excited electrically, at or near resonance, the sensor core vibrates such that all fouling matter is removed from the surface of the sensor and also such excitation prevents any growth to occur. It is particularly important that the sensor can be cleaned while submerged in the ocean, and the cleaning in no way reduces the sensor sensitivity or interferes with measurements made simultaneously while cleaing. Also, the cleaning process does not deteriorate with time as do toxic antifouling coatings. When the sensor is excited continuously below cavitation threshold, the sensor system deters all bio-fouling from the sensor surface. The cleaning of the sensor surface and the bio-fouling prevention of the sensor surface is done with the same circuitry. The difference being that the cleaning is performed by use of greater energy intensity levels than are required for the bio-fouling prevention (antifouling). The antifouling system can work on a number of different frequencies and the system can utilize any frequency required.